The invention relates to a metal-halide lamp comprising a discharge vessel with a ceramic wall which encloses a discharge space with an ionizable filling including at least Hg, an alkali halide and CeI.sub.3, and which discharge space further accommodates two electrodes whose tips are arranged at a mutual distance EA, and the discharge vessel has an inside diameter Di at least over the distance EA, and the relation EA/Di&gt;5 is met.
A lamp of the type mentioned in the opening paragraph is known from the European patent application No. 96203434.4 (U.S. Ser. No. 08/982,563, now U.S. Pat. No. 5,973,453). The known lamp, which combines a high luminous efficacy with acceptable to good color properties (inter alia a general color rendering index R.sub.a .gtoreq.45 and a color temperature T.sub.c in the range between 2600 and 4000 K) can particularly suitably be used as a light source for, inter alia, general lighting purposes. As a result of the comparatively small diameter with respect to the electrode distance and hence the discharge arc length, the discharge arc is restrained by the wall of the discharge vessel, and it is attained that the discharge arc has an approximately straight shape. This is very advantageous in connection with the Ce present, since Ce generally has a strong contracting influence on the discharge arc of the lamp. In general, it applies that a discharge arc will exhibit a greater degree of curvature in the horizontal burning position as the degree of contraction of said discharge arc is greater. It has also been found that, as a result of this geometry, the wall of the discharge vessel is subject to such uniform heating that the risk of fracture of the wall of the discharge vessel as a result of thermal stress is very small. It has further been found that said geometry also substantially counteracts the occurrence of spiral-shaped instabilities in the discharge.
By restraining the discharge arc, use is advantageously made of a good thermal conductivity of the ceramic of the wall of the discharge vessel as a means of limiting thermal stresses in the wall of the discharge vessel.
In this description and in the claims, the term ceramic wall is to be understood to mean both a wall of metal oxide, such as sapphire or dense-sintered polycrystalline Al.sub.2 O.sub.3, and a wall of metal nitride, such as AlN. These materials can very suitably be used to manufacture gastight translucent bodies. The light emitted by the known lamp has a color point with co-ordinates (x,y) which differs so much from the color point of the light emitted by a full radiator that it cannot suitably be used for indoor lighting. The collection of color points of a full radiator is commonly referred to as black-body-line (BBL). For indoor lighting purposes, it applies that only light whose color point deviates only slightly from BBL is to be considered as white light. Therefore, in general, it applies for indoor lighting applications that the color point co-ordinates (x,y) deviate maximally (0.03; 0.03) and preferably not more than (0.015; 0.015) from the BBL at the same color temperature T.sub.c.
In the known lamp, use has been made of the insight, which is known per se, that a good color rendering can be achieved if the alkali halide is used in the form of Na-halide as the filling constituent of a lamp, and that during operation of the lamp a strong broadening and reversal of the Na-emission in the Na-D lines occurs. This requires a high temperature of the coldest spot T.sub.kp in the discharge vessel of at least 1100 K (820.degree. C.).
The requirement of a high value of T.sub.kp excludes, under practical conditions, the use of quartz or quartz glass for the wall of the discharge vessel and compels the use of ceramic for the wall of the discharge vessel.
EP-A-0215524 (PHN 11.485) discloses a metal-halide lamp in which use is made of the above-described insight, and which lamp has excellent color properties (inter alia, general color-rendering index R.sub.a .gtoreq.80 and a color temperature T.sub.c in the range between 2600 and 4000 K) and hence can very suitably be used as a light source for, inter alia, indoor lighting. Said known lamp has a relatively short discharge vessel for which applies that 0.9.ltoreq.EA/Di.ltoreq.2.2, and a high wall load which, for practical lamps, amounts to more than 50 W/cm.sup.2. In said application, the wall load is defined as the quotient of the wattage of a lamp and the outer surface of the part of the wall of the discharge vessel located between the electrode tips.
A drawback of this lamp is that it has a relatively limited luminous efficacy.
Metal-halide lamps with a filling comprising not only an alkali metal and Ce, but also Sc, and with a color point which is very close to the BBL, are known per se. However, as a result of its very strong reactive character, Sc proved to be unsuitable for use in a metal-halide lamp having a ceramic lamp vessel.